Liquid scintillation counting is a generally known and widely used technique for the measurement of low energy beta emitting radionuclides in addition to gamma and alpha emitters. The liquid scintillation counter is utilized to make quantitative measurements of radioactivity by incorporating a radio-labeled analyte into solution with a liquid chemical medium capable of producing photons resulting from the kinetic interactions of nuclear decay products. There are different radionuclides, both man-made and naturally occurring, which can be experimentally employed to illustrate the molecular dynamics of a particular model by measuring their distributions in various systems. It is often desirable to use multiple radionuclides in the same experiment to follow different distributions in the same system simultaneously.
The energy of the beta particles ranges from zero energy to the maximum energy (E.sub.max) for that radionuclide. A pulse height energy distribution spectra may be recorded for the solution being analyzed. When the solution being analyzed contains a mixture of beta emitters there is an overlap of the spectra from each emitter.
Tritium has developed immense importance in various applications wherein radio-labeled components are utilized. By dissolving a sample containing the tritium in a mixture of aromatic solvent containing an organic scintillator, energy from the beta decay is transferred with reasonable efficiency to the scintillator which emits multiple photons of light proportional to energy of the beta particle. The light is detected by sensitive photomultiplier tubes. In a normal state these photomultiplier tubes have a characteristic dark noise which occurs upon the spontaneous emission of an electron within the tube. The dark noise consists of pulses generating approximately a single photon response. Thus, by utilizing multiple photomultiplier tubes and by requiring a coincidental event, beta emitters can be measured while minimizing the background. The higher the energy of the beta particle, the more photons will be produced over selected time intervals such as 20-50 nanoseconds. All photons produced within that interval are considered a pulse. The combined amplitude of the photons is known as the pulse height. The range of pulse heights as shown in FIG. 1 is a distribution of energies ranging from zero energy rising to a maximum and tailing off to the maximum energy of the beta emitting radionuclide. This distribution is known as the pulse-height energy distribution spectra.
Another phenomenon of interest to a liquid scintillation user is the phenomenon of quenching. All energy created by the beta particle is not transmitted to the fluor or otherwise is unfortunately not dissipated into the production of light. The adding of sample material to the scintillation solution introduces molecules which are not capable of producing light. The energy of the ejected beta particles therefore may be dissipated in the excitation of solvent molecules as well as by transferring energy to sample molecules. Since some of the energy transferred to sample molecules is not utilized to produce photons of light, it is not measured by the photomultiplier tubes and not recorded. This chemical quenching results in the spectrum of pulse amplitudes being smaller than the theoretical spectrum for an unquenched sample. Additionally, there is quenching caused by optical effects such as color and/or turbidity which is referred to as optical quenching.
The relative quenching of a sample may be determined by using a quench-indicating parameter (QIP) such as the Spectral Index of a Sample (SIS) or the Spectral Index of the External Standard (SIE). These methods involve counting the sample either with or without a gamma radiation source present adjacent the sample vial. The gamma radiation source generates Compton electrons in the sample solution which behave in a similar manner to beta particles. When quenching is present, the pulse height energy distribution spectra from the gamma radiation generated events will be compressed towards a lower apparent energy. By measuring the spectra change using a quench-indicating parameter (QIP) such as External Standard Ratio (ESR), Spectral Index of External Standard (SIE), H number, etc., one may obtain the degree of quenching.
Another indication of the degree of quenching is the highest energy level of the pulse-height energy distribution spectra. However, determining this endpoint energy level is difficult to accomplish in a highly accurate manner. Pulse-height energy distribution spectra tend to be uneven and occasional results fall outside the expected realm of results. The selection of the highest energy pulse recorded is not an accurate means for determining an endpoint of the spectra.
The degree of quenching of a sample is used for many purposes. To determine the actual disintegrations per minute (DPM) of a sample it is necessary to know the efficiency of the liquid scintillation counter under the particular conditions. Once the efficiency is known the actual number of counts per minute (CPM) as regarded by the counter is divided by the efficiency to determine the actual disintegrations per minute (DPM) of the sample.
For a given sample, the relationship of the degree of quenching can be equated to the counting efficiency such that the efficiency and consequently the actual disintegrations per minute may be determined from the counts per minute (CPM) based upon the degree of quenching detected.